In optical data transmission, a signal to be transmitted is sent as a sequence of light pulses over an optical fiber to a photo detector which converts the optical signal into an electronic one for subsequent processing. The most straightforward method of data transmission is to provide a different optical fiber per transmission. However, the use of a different fiber per transmission is expensive and therefore various techniques were proposed to allow multiple signals to be transmitted over a single fiber. The two most common techniques are Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM).
In TDM, separate input signals are carried on a single fiber by allocating time transmission windows. The input signals are fed to a multiplexer which schedules use of the optical fiber so that each input signal is allowed to use the fiber in a specific time slot. At the receiver, synchronization techniques are used to ensure that the different input signals are sent on to the appropriate destination.
In WDM, the fiber is shared by sending each input signal at the same time, but on a different carrier wavelength channel, for example a first signal could be transmitted using a carrier wavelength of 1539 nm and another signal is transmitted using a carrier signal of 1560 nm.
All modern optical data transmission utilizes TDM, with core transmission additionally utilizing WDM. In core data transmission, individual signals rates of up to 100 Gbit/sec are achieved through the use of TDM; these individual signals are then multiplexed onto a signal fiber through WDM in order to further enhance the transmission rate.
Considering WDM in greater detail, a grid of wavelengths is specified by the International Telecommunication Union (ITU) so that compliant equipment from different manufacturers can operate together. The ITU has specified a number of Dense Wavelength Division Multiplexing grid sizes at 12.5 Ghz, 25 Ghz, 50 Ghz and 100 Ghz. 50 Ghz is currently the most popular channel and, using the DP-QPSK modulation format, it is possible to fit a 100 Gbit/s signal within a single channel in the 50 Ghz grid. However, research into optical transmission beyond 100 Gbit/s has shown that higher spectral efficiency formats have to be used, or the spectral width of the signals must be increased to support 400 Gbit/s or 1 Tbit/s transmission. Utilizing modulation formats with higher spectral efficiencies limits the distance the signal can propagate due to OSNR penalties, and increasing the spectral width means that the signal can no longer fit within the widely deployed 50 Ghz ITU grid. To overcome these problems, flexible grid or Flexgrid networks have been proposed. In this scheme, arbitrary sized wavelength blocks can be specified by the network owner, thereby accommodating new bit rate services.
In order to transmit signals by WDM, whether on the fixed grid or flexible grid network, two signals having different carrier wavelengths must be multiplexed onto the same optical fiber. Providing the carrier wavelengths are sufficiently different, the signals will not interfere with each other.
Optical fibers carrying the multiplexed signals meet other optical fibers carrying different multiplexed signals at a network node. A node generally consists of two parts: add/drop (A/D) and bypass routing. The add/drop component is arranged for dropping optical signals at certain wavelength channels for transmission to receivers associated with the node. The add/drop component is also arranged for adding optical signals at certain wavelength channels to the optical signals already carried on the optical fibers. In contrast, the bypass routing component is arranged for routing wavelength channels received via node input fibers to the correct node output fibers to enable onward transmission to the desired adjacent nodes. The input optical signals are demultiplexed into the individual wavelength channels by the bypass router, switched to the desired output and re-multiplexed for onwards transmission. The bypass routing component treats the individual wavelength channels separately: one wavelength channel may be routed through the node differently to another wavelength channel. One known component for bypass routing is a Reconfigurable Optical Add-Drop Multiplexer (ROADM).
The above-mentioned nodes at which these fibers meet are classified according to the number of fiber directions that converge at that node. For example, if optical fibers deliver data to and from North, South and West then the node at which these fibers meet is a degree three node. Current telecommunications networks comprise a single optical fiber for data transmission in a given direction. It will be appreciated that six fibers converge at a degree three node if the network comprises a single fiber per direction: one fiber for data transmission from North, one fiber for data transmission to North, etc.
However, due to the ever increasing bandwidth demands on telecommunications networks, it is anticipated that multiple fibers in one or more directions will be required in the near future. Accordingly, many more fibers will converge at a node of a given degree. For example, a degree three node in a “multi-fiber” network may comprise six or more fibers. In a “multi-fiber” arrangement such as this, it is envisaged that a number of independent channels or superchannels will be spread across the multiple fibers, the number of channels or superchannels carried on any one of the fibers being variable in accordance with the optical spectrum and/or the network architecture.
It is envisaged that improvements to current node technology will be required in order to cope with the demand of multi-fiber networks. In particular, known bypass routing components typically demultiplex WDM optical signals via Wavelength Selective Switches (WSSs). These switches disperse optical signals by means of a diffraction grating or the like, which enables each wavelength channel within the optical signals to be treated differently, i.e. blocked or allowed to pass through the switch. A 1×N WSS enables any wavelength channel that is received by the switch to be output at any of the N output ports thereof. Whilst these known bypass routing components function effectively for single optical fiber inputs, it would be expensive to provide sufficient WSS capacity within a multi-fiber network. In particular, if the multi-fiber network is configured to transmit N independent wavelength channels or superchannels across F optical fibers then any one of the optical fibers could carry optical signals comprising N independent wavelength channels. Accordingly, F 1×N WSS switches would be required for demultiplexing the optical signals (i.e. one for every fiber) in order to cope with all possible wavelength channel distributions. Whilst it would be possible to construct a bypass router in this way, WSSs are expensive, particularly those having a large number of output ports.